You sold the stock, you owe the tax. There really isn't any way around that now that the sale is done.
Now would be a fine time to realize any deductible capital losses. If you haven't yet invested the cash you received from the sale of this stock, get on that now.
Sell my place this year and use that 100k loss as a deduction - but apparently this is disallowed by the IRS. :( Any creative ways to leverage this loss?
As you note, you may not deduct capital losses from your primary residence.
-Put in ~15k worth of repairs/upgrades into my place, which I can deduct if I rent it out? Or some other rental strategy..?
If you convert the home to a rental you would be able to deduct some repair expenses, but you'd first have to move out into a different place. Seems like a lot to squeeze into the last three months of the year, just to save a couple grand in taxes. I guess if you were planning to rent out your current condo in the next few years anyway, there might be some marginal advantage to doing it in this very high income year.
Buy an online business for $XXk and use that as a deduction (while trying to turn it into income)?
I wouldn't expect that the purchase price of an operating business would be a deductible expense. That sounds more like an investment than an expenditure.
Something charity related?
If you had donated the shares prior to selling them, you would have been able to deduct the entire value of the donated shares. Since you already sold them this is no longer an option. Donating cash to a charity could lower your taxes if it pushes you over the line for itemized deductions.
File state taxes in December to get the federal deduction this year?
I thought I heard they were banning the practice of deducting early payments for taxes due in a future year, but I could be wrong about that. My state doesn't believe in income tax so it's not too relevant to me.
-Quit/Pause my job until 2019 since my income is essentially super taxed this year? Am I thinking about that right??
I don't think you're thinking about this right. The long-term gains go on top of the regular income tax. You're taxed the same on the job income whether you have a capital gain or not. What could happen, only if your capital gains income goes across bracket lines (i.e. some is taxed at 15% and some is taxed at 20%), is that reducing your job income by $1,000 would move $1,000 of that 20% capital gains into the 15% bracket. Would you decide that spending your time going to work was no longer worth the effort if your taxes went up by 5%? Maybe, but likely not.